Revealing the Contribution of Cu Vacancies in Nonstochiometric Cu _2–x Se to Ethanol Selectivity in CO ₂ Electrolysis

Cu-based catalysts are unique in promoting the C–C coupling reaction toward forming multicarbon products in the electrocatalytic carbon dioxide (CO2) reduction reaction. However, achieving a high selectivity toward a desirable product remains a grand challenge. Here, we show that Cu vacancies in nonstoichiometric Cu2–xSe play a paramount role in ethanol selectivity. Electron paramagnetic resonance in combination with Raman spectroscopy characterization results shows the presence of Cu vacancies, which are absent from stoichiometric copper chalcogenides Cu2Se and Cu3Se2 and a Cu-oxide-based catalyst, Cu/Cu2O. In situ Raman and attenuated total reflection-surface-enhanced infrared absorption spectroscopy characterization results reveal that Cu vacancies in Cu2–xSe significantly promote the formation of species *OCCOH and *OC2H5, which are the important intermediates for forming ethanol, while inhibiting the hydrogen evolution reaction. This work highlights the importance of cationic vacancies of metal chalcogenide compounds in product selectivity during the CO2 electrolysis.